The present invention relates generally to an inspection apparatus for detecting surface distortion in a sheet of material, such as a glass plate, for example. More particularly, the present invention concerns an apparatus for determining the amount of surface distortion in a glass plate as the glass is carried past the inspection apparatus.
In the known methods of making and shaping glass, defects may inadvertently be produced in the glass which render the glass optically imperfect. Defects may also be produced in the glass during subsequent manufacturing operations such as during a tempering operation, for example. Among the optical imperfections that may be produced is surface distortion. Surface distortion, as the term is used herein, generally refers to variations in surface flatness, i.e. concave and convex portions.
Surface distortion in glass causes the glass surface to reflect a distorted image. For example, convex portions shrink the image and concave portions magnify the image. When excessive distortion is present, the distorted images detract from the architectural beauty and are therefore not desirable.
In addition to other methods, one approach which has been proposed for detecting surface distortion of a piece of glass is disclosed in U.S. Pat. No. 3,857,637 to Obenreder. The Obenreder patent discloses an inspection apparatus which utilizes a light source and a position sensing photodetector for detecting concave and convex portions on the inspected surface and the amplitude of such portions. The light source, such as a continuous laser, directs a beam of light on the upper surface of a glass plate traveling at a constant speed along a predetermined path relative to the light source. The position sensing photodetector is mounted to detect the portion of the light beam reflected by the upper inspected surface of the glass plate.
If the inspected surface is flat, the reflected portion of the beam will be received by the photodetector along a predetermined reference line. When the light beam is reflected from concave or convex portions in the surface, the reflected beam will be displaced from this reference line. The inspection apparatus includes means responsive to the detector output signals to produce a surface flatness profile showing the nature of the surface curvature, i.e. concave or convex, and the amplitude of the curvature. While such an apparatus is capable of determining the surface flatness of a sheet of glass, there is no means provided for analyzing the data for determining whether the distortion level of an inspected sheet of glass is unacceptable. Also, such an apparatus is subject to errors from changes in glass position during the measurement process.